A plasma process is a technique indispensable to the manufacture of semiconductor devices. In recent years, further miniaturization of semiconductor devices constituting LSI has been required to meet demand for high-integration and high-speed of LSI.
To do this, recently, there has been proposed a microwave plasma processing apparatus which is capable of generating plasma with a low electron temperature and a high plasma density, as compared with a capacitively coupled plasma processing apparatus or an inductivity coupled plasma processing apparatus used in a conventional plasma process.
There is known a plasma processing apparatus which generates a surface wave plasma having a high plasma density at a low electron temperature within a processing vessel by transmitting a microwave through a coaxial tube to radiate the same into the processing vessel and exciting a plasma generation gas using an electric field energy of a surface wave of the microwave.
However, such a plasma processing apparatus has a structure in which, in order to irradiate the processing vessel with the microwave through the coaxial tube, a ceiling portion of the processing vessel is configured such that a quartz dielectric plate is inserted between the surface wave plasma and an antenna, and a processing gas is supplied into the processing vessel through a sidewall of the processing vessel. In this way, since the gas is supplied from a portion other than the ceiling portion, the flow of the gas cannot be controlled. This fails to achieve good plasma control.
To address this, there is proposed a technique which installs a shower plate on a bottom surface of an antenna, and introduces a processing gas into a processing vessel along a path directed vertically downward from the shower plate. The shower plate is formed of a dielectric. The processing gas is supplied through a plurality of gas supply holes penetrating through the shower plate in its thickness direction. A leading end portion of the gas supply hole has a narrow orifice shape to prevent reverse diffusion of the gas from the inside of the processing vessel to the gas supply hole. This arrangement forms a vertical gas stream inside the processing vessel and uniformly supplies the processing gas, thereby forming uniform plasma.
However, for example, in the plasma processing apparatus including the antenna and the shower plate, an electron temperature of plasma in a region in the vicinity of the shower plate is higher than an electron temperature in a position spaced apart from a surface of the shower plate due to the surface wave plasma. As such, in a film formation process, a raw material gas, such as a monosilane gas (SiH4), is excessively decomposed in the vicinity of the shower plate so that a film of the decomposed gas is deposited in the shower plate. This may clog the leading end of the gas supply hole having an orifice shape. Such a clogging causes a deviation in the flow of the processing gas inside the processing vessel.
In order to prevent excessive decomposition of the raw material gas, the output of the microwave supplied to the antenna is required to be decreased. However, the decrease of the output of the microwave insufficiently excites the plasma generation gas. This fails to stably form plasma. Also, in terms of productivity, it is required to efficiently excite the plasma generation gas and improve a film forming rate in a film formation process, for example.
Under these circumstances, the present inventors have found that, to prevent the excessive decomposition of the raw material gas requires installing a drooping portion in the bottom surface of the shower plate, the drooping portion being formed to protrude downward while spreading outward as it goes from the top end to the bottom end thereof, supplying the raw material gas such as a monosilane gas into the processing vessel through gas supply holes formed in the drooping portion, and supplying the plasma generation gas outward of an outer surface of the drooping portion. By doing so, the raw material gas does not pass through a region in which the electron temperature of the surface of the shower plate is high so that the excessive decomposition of the raw material is prevented. Since the outer surface of the drooping portion spreads outward as it goes from the top end toward the bottom end thereof, the microwave is reflected in a lateral direction or an upwardly inclined direction from the outer surface of the drooping portion. As a result, it is considered that the electric field intensity in the vicinity of the outer surface of the drooping portion is increased, so that the plasma generation gas can be efficiently excited to thereby generate plasma in addition, the present inventors have actually performed a confirmation test using the shower plate equipped with the drooping portion. The test shows that it is possible to prevent the excessive decomposition of the raw material gas and to appropriately excite the plasma generation gas.
However, it was confirmed that an orifice portion of the gas supply hole was still clogged even when the shower plate equipped with the drooping portion is used. As a result obtained by inspecting a material that lead to the clogging, it was confirmed that the material was a product derived from the raw material gas or the plasma generation gas, which is called as SiHx or SixNy. The reason for this may be that the electron temperature is increased in the vicinity of the orifice portion of the gas supply hole by the microwave surface wave on the surface of the shower plate, and SiH2 or Si is generated as electrons collide with the monosilane gas in the vicinity of the orifice portion, or SiN is generated as radicals of the plasma generation gas such as nitrogen gas collide with the monosilane gas. Thus, it is required to control the electron temperature in the vicinity of the orifice portion of the gas supply hole or suppress the generation of the product.